A silicon chip, or integrated circuit (IC), is the core element of an electronic device and usually comes in packaged form. With the development of manufacturing technology and requirement of compact design for end products, various packaging methods were invented to meet the demand. At most of the time, silicon chips are sealed within a protection material such as epoxy resin. There are certain cases, particularly when the silicon chip is a sensor device such as a fingerprint sensor chip, that the silicon chip needs to be mount on a substrate and has the surface exposed. Meanwhile, for a fingerprint reader device, the thickness of the packaged sensor must be as low as possible. Hence, the technique of bonding the silicon chip to the substrate plays a very important role. The bonding needs to ensure good circuit connectivity formed therebetween, and to be rigid enough to sustain the fingerprint reader sensor with shear force exerted by a finger.
Flip chip technology is a suitable means for better interconnecting between the die and the PCB. Processing a flip chip is similar to the conventional IC fabrication, with a few additional steps. Please refer to FIG. 1. Near the end of the manufacturing process, the attachment pads 2 of a chip 1 are metalized to make them more receptive to solders. This typically consists of several treatments. Small dots of solder balls 3 are then deposited on each metalized pad 2. The chips 1 are then cut out of the wafer as normal. To attach the flipped chips 1 into a PCB 4, the chip (die) 1 is inverted to bring the solder balls 3 down onto connectors 5 on the underlying PCB 4. The solder balls 3 are then re-melted to produce an electrical connection, typically using a thermosonic bonding or alternatively using a reflow solder process. This also leaves a small space between the chip's circuitry and the underlying mounting. In most cases an electrically-insulating adhesive 6 is then underfilled to provide a stronger mechanical connection.
One challenge of the flip chip technology is heat dissipation for thermal stress in the chip 1. The adhesive 6 is used as a heat bridge to ensure that the solder balls 3 are not stressed due to differential heating of the chip 1 and the PCB 4. The adhesive 6, distributing the thermal expansion mismatch between the chip 1 and the PCB 4, prevents stress concentration in the solder balls 3 which would lead to premature failure. When fan-outs of logic gates of a chip increases and associated pads for soldering become more and tiny, the efficiency of heating dissipation of such adhesive for the solder balls get worse. In other words, the tiny solder balls will easily get hurt to cause open-circuit failure since limited quantity of the adhesive applied to take away unnecessary heat. Meanwhile, for a fingerprint reader sensor chip, the binding ability between the chip and the PCB may not sufficient to resist the force exerted by a finger.
A better solution to settle the problem could be available from the U.S. Pat. No. 5,045,921. Please see FIG. 2. An electronic pad array carrier IC device 10 for mounting on a PCB is disclosed. It has a thin and flexible “tape” substrate 11 which has a number of traces 12. The substrate 11 can withstand relatively large lateral mechanical displacement. An integrated circuit die 13 is mounted in proximity with or on the substrate 11. The substrate traces 12 are provided at their outer ends 14 with solder balls 15 for making connections to the PCB. A package body 16 covers the die 13. Alternatively, a carrier structure may be provided around the periphery 17 of the substrate 11 to add rigidity during handling, testing and mounting, but which may also provide the stand-off function. The thin, flexible substrate 11 can absorb a relatively large lateral or even vertical mechanical displacement over a rather large package area that may accommodate as few as 20 or as many 500 or more connections. The solder balls 15 may be joined to a via 18 through the substrate 11 at least partially filled with electrically conductive material to permit back side testing before or after mounting of a package to the PCB. Additionally, a heat sink structure 19 may be directly bonded to the die 13 in the pad array carrier IC device 10. Although '921 was an innovative invention to provide bonding between a die and a PCB with a special tape substrate, it is the tape substrate causes complexity in mounting and extra cost.
Therefore, an improved method for mounting a chip on a PCB is still desired. More particularly, the binding ability between the chip and the PCB should resist external force exerted onto the chip.